FIG. 1 is a schematic illustration of nominal divisions or categorizations of the angular field of view of a human eye. The highest concentration of photoreceptor cells in the retina is in the region of the macula, and more particularly within the subregion of the macula called the fovea. The central optical elements of the eye, i.e., the cornea and the lens, together with the macula define a macular visual field 107 that typically spans an angle of about 18° to 19°; the macular visual field is considered the center of the overall visual field. Note that the depiction in FIG. 1 is a two-dimensional representation of a three-dimensional visual field. The macular visual field therefore corresponds roughly to a cone with an included angle of about 18°. Other two-dimensional depictions included in the present disclosure shall be similarly interpreted as representing three-dimensional, often roughly conical arrangements as appropriate. The macular angular region 107 can be sub-divided into roughly concentric regions: the fovea (the central, inner region of the macula with a corresponding field of view having an included angle of about 5° and defining the center of the visual field), which is embedded within the parafovea (with a corresponding ring-like field of view between about 2.5° and about 4° from the center of the visual field), which in turn is embedded in the perifovea (with a corresponding ring-like field of view between about 4° and about 9° from the center of the visual field). The roughly 18° macular angular field 107 corresponds to a macular region 108 on the retina 109 (FIG. 3) that is about 5.5 mm across. The macular region 108 typically is considered the central retina portion in the present disclosure, while the rest of the retina 109 is considered the peripheral retina portion; a different definition of the central and peripheral retina portions can be employed if suitable, desirable, or necessary. For example, in some examples only the fovea is considered the central retina portion while the remainder of the retina (including the parafovea and perifovea) is considered the peripheral retina portion; in other examples the fovea and parafovea together are considered the central retina portion while the remainder of the retina (including the perifovea) is considered the peripheral retina portion.
Referring again to FIG. 1, the periphery 90 of the visual field extends between about 9° and about 100° to 110° from the center of the visual field. The peripheral field portion 90 can be subdivided into the near periphery 90a (between about 9° and about 30° from center), the mid periphery 90b (between about 30° and about 60°), and the far periphery 90c (between about 60° and about 100° to 110°). Those subdivision are arbitrary and are provided only for convenience of description.
FIG. 2 is an example of photoreceptor density as a function of view angle for rod photoreceptors (curve 118) and cone photoreceptors (curve 119) in a human retina. The minimum in rod density that coincides with the maximum cone density corresponds roughly to the location of the fovea on the retina 109. The other minimum in the rod density corresponds to the so-called blind spot, where the optic nerve enters the retina. Each curve in FIG. 2 represents a specific cross section across the eye; a different cross section might miss the blind spot so that the curve 118 would exhibit only a single minimum in the rod density curve 118. An alternative definition of the central and peripheral retina portions can be based on relative densities of rods versus cones. That portion of the retina having a cone-to-rod ratio sufficiently high would be considered the central retina portion, and the rest of the retina (with a lower ratio) would be considered the peripheral retina portion. Note that the alternative definitions (macula-based or rod/cone-based) typically nearly coincide.
The macula (and the fovea in particular) has the highest photoreceptor density and provides the sharpest vision at the center of the visual field. Central vision typically is provided primarily by cones; peripheral vision is provided primarily by rods. It has long been observed and well understood that cones provide color vision and high visual acuity under relatively high levels of illumination, while rods provide more sensitivity at relatively low levels of illumination. Ganglia present in the peripheral retina portion also provide enhanced sensitivity to detecting the presence or direction of motion.
A leading cause of vision loss is macular degeneration, with incidence rates estimated to be as high as 1.5% among people over 40 years old. The central portion of the retina 109, i.e., the macula 108, deteriorates leading to blurred or no vision in the center of the visual field. Loss of central vision can make it hard to recognize faces, drive, read, or perform other activities of daily life. Peripheral vision typically is unaffected.